Electric vehicles, hybrid electric vehicles (HEV's), plug-in hybrid electric vehicles (PHEV's) and other vehicles that utilize an electric machine, such as an electric motor, may be configured to use the electric machine to provide regenerative braking to at least assist in stopping the vehicle. Regenerative braking may provide a number of advantages over using a friction braking system exclusively. For example, the use of regenerative braking, whereby an electric motor provides negative torque to the vehicle wheels, reduces wear on the friction elements of the friction braking system. In addition, during regenerative braking, the motor may function as a generator, producing electricity that may be used immediately or stored in a storage device, such as a battery.
During Regenerative Braking the transmission may downshift as the vehicle speed decreases. In vehicles having a conventional powertrain, including an automatic transmission, the torque converter may be used for torsional isolation during the downshift. The torque converter clutch is slipped during power-on upshifts and coasting downshifts so as to impart a smooth feeling to the vehicle occupants. This smooth feeling is due to the removal of a hard coupling to the driveline during shifting. The slipping in the torque converter clutch usually increases input speed above the turbine, and pump and line pressures are not affected. During coasting shifts, relatively small negative torques are applied to the input—for example by shutting off one or more cylinders in a variable displacement engine—and any slip that is generated is well controlled.
During large negative torque regen downshifts—i.e., a transmission gear downshift during a regenerative braking event—the drawbacks associated with using a torque converter to isolate driveline disturbances is increased, largely from the lower speeds of the transmission pump that could occur. Lower pump speed adversely affects controllability of the torque converter clutch to control negative slip. For this reason, the torque converter often remains locked during regen shifting. This locked torque converter clutch increases the chance of driveline oscillation when the oncoming shifting clutch finally stops slipping after the ratio change. The landing of this clutch at synchronous speed using pressure control may be difficult.
One alternative to using pressure control is to use the electric motor to damp or otherwise control the driveline oscillations. The use of active motor damping after the shift may reduce driveline oscillations; however, there is a lag between the end of the shift and when the active motor damping begins to control the driveline oscillations. During this lag, the driveline oscillations may be noticeable, resulting in a poor shift quality being perceived by the vehicle occupants.